Lithium-ion batteries quickly occupied the high-end rechargeable battery market after the first commercialization of lithium-ion battery by Japanese Sony Company in the early nineties in the last century. Lithium-ion battery, as a new type of clean energy, has characters of high specific energy density, high working voltage, long cycle life and no memory effect, etc. Currently, lithium-ion batteries are considered as the most promising mobile energy storage technologies in the mobile electronics, electric vehicle industry and other recycling energy systems.
At present, lithium-ion batteries generally use graphite-based carbon materials as an active material for negative electrode, such materials have a small change in volume in the charge-discharge cycle. However, such carbon materials have low battery capacity. For example, high crystallinity graphite has a theoretical capacity of 372 mAh/g. However, when using a material having high discharge capacity, such as silicon, tin, or a silicon-tin alloy, as an active material for negative electrode, the volume change is large and the materials deteriorates easily. For example, there is a volume expansion rate which is up to 300% during lithium insertion of a silicon-based active material, the expansion stress causes the structure of the silicon-based material to be crushed, thereby breaking the conductive connection between the active material of the electrode material and the current collector and thus degrading the electrode pole piece.
In order to solve the problems such as deformation and deterioration of the battery caused by the cycle volume expansion of the negative electrode non-carbonaceous active material, it is necessary to develop a binder having high adhesive strength and capable of uniformly dispersing the expansion stress to realize the cycle stability of negative electrode of high-capacity non-carbonaceous active material.
Patents CN103242595 and CN101243566A respectively disclose an inorganic nano-particle composite binder, which improves the tensile strength by adding nanometer inorganic fillers (such as nano-silica) and carbon nanotubes and is used as a binder for negative electrode of non-carbonaceous active material having high capacity and high volume expansion rate. However, although the size and surface morphology of the nanoparticles are controlled in advance, agglomeration of the inorganic nano-particles during the film formation of the binder inevitably occurs in the physical compound method, which will affect the uniform stability of pole piece. Patent CN102875722A discloses a strong adhesive type inorganic-organic composite binder prepared by a method based on in-situ emulsion polymerization of carbon nanotube. However, at present, there is not yet a functional adhesive having high volume expansion rate which has excellent adhesive strength and meanwhile exhibits higher flexibility, and is particularly suitable for silicon-based negative electrode active materials.
The incorporation of inorganic nanoparticles into an inorganic-organic composite emulsion can improve the film-forming property of the emulsion and enhance the mechanical properties of latex film. At present, inorganic-organic composite emulsions are mainly prepared by a simple physical blending method, and the main problem of this method is that inorganic nano-particles are difficult to disperse uniformly in aqueous media. To this end, by in-situ emulsion polymerization, uniform uniform coating of the inorganic nano-particles by polymers is achieved to prepare core-shell particles.
Patent CN1944479A discloses a polyacrylate composite emulsion of inorganic-organic composite latex particles having a core/shell structure for pressure-sensitive adhesives. The latex particles composing the composite emulsion has a core-shell structure with silane cross-linking agent surface-modified nano-silica as the core and the copolymer of acrylate and acrylic acid monomers as the shell, has a solid content of 30-40%, a viscosity of 1-6 MPA·S, a particle size of the 250-700 nm, and a particle size distribution index of 0.005-0.15. The preparation method thereof comprises the following steps: preparing nano-silica alcohol sol, surface modifying nano-silica and preparing nano-silica-polyacrylate composite emulsion. The composite emulsion coating film was dried to obtain acrylate pressure-sensitive adhesive. By this, the initial adhesion and cohesion performance of pressure-sensitive adhesive are improves, nano-silica particles are dispersed in the polyacrylate pressure-sensitive adhesive matrix, and the thickness of core layer and shell layer latex particle can be controlled. However, in the process of preparing inorganic-organic composite emulsion by this method, small-molecule emulsifier is used, and most of the nano-silica particles are still agglomerated due to non-uniform dispersion. Meanwhile, the small molecule emulsifier affects the solvent resistance of the adhesive.